Air-polluting emissions are an undesirable by-product of the operation of gas turbines. The primary air-polluting emissions produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide and unburned hydrocarbons. It is well known that oxidation of molecular nitrogen in air-breathing engines is dependent upon the maximum hot gas temperature in the combustion system reaction zone. The rate of chemical reactions forming oxides of nitrogen is an exponential function of temperature. Consequently, if the temperature of the hot combustion gas is controlled to a low level, thermal NO.sub.x will not be produced.
A typical and preferred method of controlling the temperature of the reaction zone of a gas turbine combustor below the level at which thermal NO.sub.x is formed includes pre-mixing the fuel and air to a lean mixture prior to combustion. The thermal mass of the excess air present in the reaction zone of a lean, pre-mixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where NO.sub.x is not formed. However, by lowering the temperature in the reaction zone to that level, the fuel/air mixture strength is reduced to a level close to the lean flammability limit for most hydrocarbon fuels. As a consequence, lean, pre-mixed combustors tend to be less stable than more conventional diffusion flame combustors and do not provide adequate turn-down for operation over the entire load range of the gas turbine. Stability for operation over all load conditions required for gas turbine operations, with minimum emissions of air pollutants in the gas turbine exhaust can be achieved with proper pre-mixing capability.
Dual fuel injectors with pre-mixing capability for heavy duty industrial gas turbines are known. For example, in U.S. Pat. No. 4,701,124, pre-mixing capability is provided. However, that design is not afforded in a compact envelope, nor does it achieve uniform fuel/air mixture in a manner which tolerates maldistribution of inlet air flow/velocity while maintaining a uniform fuel/air mixture strength at the injector discharge, which is necessary to achieve low emissions combustion.